The present invention relates in general to delta-sigma modulators and in particular, to cascaded noise shaping circuits with low out-of-band noise and methods and systems using the same.
Delta-sigma modulators are particularly useful in digital to analog and analog to digital converters (DACs and ADCs). Using oversampling, a delta-sigma modulator spreads the quantization noise power across the oversampling frequency band, which is typically much greater than the input signal bandwidth. Additionally, the delta-sigma modulator performs noise shaping by acting as a highpass filter to the noise; most of the quantization noise power is thereby shifted out of the signal band.
The typical delta-sigma modulator in an ADC includes an input summer which sums the analog input signal with negative feedback, an analog linear (loop) filter, a quantizer and a feedback loop with a digital to analog converter (feedback DAC) coupling the quantizer output and the inverting input of the input summer. A delta-sigma DAC is similar, with a digital input summer, a digital linear filter, a digital feedback loop, a quantizer and an output DAC at the modulator output. In a first order modulator, the linear filter comprises a single integrator stage; the filter in higher order modulators normally includes a cascade of a corresponding number of integrator stages. Higher-order modulators have improved quantization noise transfer characteristics over modulators of lower order, but stability becomes a more critical design factor as the order increases. For a given topology, the quantizer can be either a one-bit or a multiple-bit quantizer.
In cascaded delta-sigma modulator topologies, commonly referred to as MASH (multi-stage noise shaping) modulators, multiple delta-sigma noise shaping loops are cascaded to produce high noise attenuation in the signal band of the modulator noise transfer function (NTF) while maintaining modulator stability. In particular, the typical MASH modulator includes two or more cascaded noise shaping loops, each having a loop filter of a given number of filter stages and a quantizer. The quantized output of one noise shaping loop drives the input of the next noise shaping loop in the cascade such that, except for the first noise shaping loop, the input of each noise shaping loop is the quantization error from the previous noise shaping loop in the cascade. The output of each noise-shaping loop is also passed through error cancellation circuitry that cancels the quantization error from all but the last noise-shaping loop in the cascade. The noise shaping of the quantization error output from the last stage of the cascade is therefore approximately nth-order, where n is the total number of loop filter stages in the cascaded noise shaping loops.
Current state of the art conventional MASH modulator topologies are capable of providing signal band noise attenuation in the NTF on the order of xe2x88x92150 dB. Additionally, MASH modulators are less susceptible to DAC non-linearity in data converter applications and are generally more stable than single-loop modulators, especially when based on low-order, proven-stable individual noise shaping loops. However, conventional MASH modulator topologies also have significant drawbacks. For example, conventional MASH modulator topologies typically achieve high signal band noise attenuation at the expense of increased out-of-band noise gain. More recently, however, out-of-band noise has become a more troublesome problem that must be addressed, especially in such applications as high performance data converters. Consequently, substantial efforts typically must be made. For example, attenuating out-of-band modulator noise at the system level often requires more precise filtering and complicated clocking schemes, which add expense and complexity to the system.
In sum, new noise shaping methods and topologies are required which not only provide the high signal band attenuation typical of MASH converters, but which also significantly attenuate out-of-band noise at the same time.
According to the inventive concepts, cascaded delta-sigma modulation circuits and methods are disclosed which provide substantial out-of-band attenuation. According to one particular embodiment, a delta-sigma data converter is disclosed which includes a first quantizer responsive to outputs of first and second loop filters, the first quantizer introducing a quantization error during quantization. The first loop filter is responsive to an output of the first quantizer. A second quantizer quantizes an error data stream representing the quantization error introduced by the first quantizer. The second loop filter includes an integrator for integrating a quantized error data stream output from the second quantizer. An analog output path responsive to the output of the first quantizer and an output of the integrator generates the converter output.
In an exemplary delta-sigma modulator application of the inventive application, a first quantizer is driven by the outputs of both first and second loop filters. The first loop filter filters the converter input signal and feedback from the first quantizer. The second loop filter receives the quantization error introduced by the first quantizer after quantization by a second quantizer This topology advantageously drives down out-of-band noise. In contrast, the, quantization noise from each noise shaping loop of a conventional MASH topology is only fed-forward to the input of the next noise shaping loop in the cascade and the corresponding cancellation logic.
Delta-sigma modulators embodying the inventive principles are suitable for use in data converters in which the output of the first quantizer drives a first digital to analog conversion path and an integrated quantized quantization error tapped from the second loop filter drives a second digital to analog conversion path. The output of the second loop filter is differentiated to generate a noise transfer function (NTF) zero at DC (i.e., at zero frequency) hereby driving-down low frequency components of the resulting analog-converted quantization error and then summed with the output of the first conversion path to generate an analog output signal with low out-of-band noise.